The demand function for a product is given by p = 4000 ln (x =10), where p is the price per unit in dollars when x units are demanded.
(a) Find the rate of change of price with respect to the number of units sold when 40 units are sold.
(b) Find the rate of change of price with respect to the number of units sold when 90 units are sold.
(c) Find the second derivative to see whether the rate at which the price is changing at 40 units is increasing or decreasing.